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Nice tank, sadly when this happened to my stars, they slowly perished into nothing. He may cut the leg off to try and save himself, but that takes a lot of energy. Hope for the best, but I don't believe you can do anything.
Fromia and linckias are super tough to keep long term. How long did you have him? He coulda been going downhill before you bought him. I doubt he will recover. IMO they're not an organism that should be sold for aquariums due to very limited long term survival stories